A diffraction element of the type used for authentication purposes is described in Swiss patent 00805/88-4 which corresponds to U.S. Pat. application Ser. No. 311,596 now U.S. Pat. No. 4,984,824. This diffraction element is provided with relief structures containing authenticity information, for example in the form of images, the colors and luminosity of which depend on the angle of observation, and which can therefore not be copied by means of conventional copying machines. The relief structures can be provided with cross-sectional configurations of known periodic functions with spatial frequencies of over 10 lines per millimeter which are effective for the diffraction of visible light. Limits imposed in manufacture restrict the practically usable range to approximately 2500 lines/mm. In addition, cross-sectional forms with aperiodic functions containing local spatial frequencies from that range, such as matte structures, can be used. The height difference of these relief structures is typically selected to lie between 50 nm and 10,000 nm.
A diffraction element known from Swiss patent 594,936 is provided with machine-readable, diffraction, optically-encoded markings that are difficult to copy. They are embossed in the form of microscopic relief structures into a thin thermoplastic support which can be applied on paper, for example, they are protected by means of an optically effective coating material, and they diffract incident light.
Pairs of optically-encoded markings are known from U.S. Pat. No. 4,108,367. It discloses a token for use in a vending machine which consists of a plastic card having at least one pair of side-by-side holograms embossed on it. A token reader guides the card into the path of a readout beam. Two photo-sensors located at predetermined positions receive first order beams diffracted from the illuminated holograms and convert them into output electrical signals. Electronic circuitry, including a comparator, enables the authenticity of the card to be determined by comparing the electrical signal pairs.